The present invention relates to a miniature electric motor such as a stepping motor, more particularly to a rotor thereof and a method for producing the rotor.
Miniature electric motors are widely used in various fields of precision equipment such as compact cameras, audio visual system such as video tape recorders, and automotive electronic equipment. A stepping motor, a brushless motor or the like may be exemplified as a miniature motor having a structure in which permanent magnets are used in its rotor. For instance, the stepping motor is widely used as a drive source for various type electronic equipment such as a printer.
Such a miniature motor is provided with a stator mounted within a casing and a rotor which faces the stator. Permanent magnets are provided in the rotor so that the rotor may be attracted and rotated by magnetic forces produced by the stator.
It is general that the overall rotor body (i.e., rotor yoke) mounted on a rotary shaft is molded with magnets into a one-piece structure, or otherwise a magnetic layer is adhered with adhesives onto an outer circumferential surface of the rotor body made of metal to form a double structure.
By the way, recently, there has been an increased demand to provide a miniature electric motor which has a high torque power and which is of a power saving type. For enhancing the torque, there are methods to increase current and to use permanent magnets, which generate strong magnetic forces, in the rotor.
The former method, i.e., a method to increase current, has to be an intrinsic limit. This could not meet the requirement of saving the power. For this reason, recently, rare earth plastic magnets which may produce strong magnetic forces with a small amount of current for generation of high torque power have been frequently used in the rotor instead of ferrite magnets that have been generally conventionally used.
However, the rare earth plastic magnets are several times more expensive than the ferrite magnets, so that it is desirable to reduce the amount of the material as much as possible.
In case of the above-described one-piece type rotor, it is relatively easy to obtain a high precision in coaxiality between the rotary shaft and the rotor. In addition, it is possible to enjoy the advantage that the number of the assembling steps therefor may be reduced because of the one-piece structure.
However, the conventional one-piece structure suffers from a disadvantage that a large amount of expensive rare earth plastic magnets is needed. It should be noted that there is a method in which a space is formed in an interior of the rotor body to thin its layer in order to reduce the amount of the used rare earth plastic magnets. However, the rare earth plastic magnets are fragile so that a mechanical strength of the rotor body is low, resulting in easy generation of cracks or breaks.
In contrast thereto, in case of the above-described double structure rotor, a difference in a thermal expansion coefficient between the rotary body made of metal and the magnetic layer composed of rare earth plastic magnet is large (for example, several tens times to several hundreds times). A stepping motor or the like which is used in a wide temperature range (for example, from -40.degree. C. to +120.degree. C.) and under an environment where the temperature is widely changed suffers from a problem that a separation or crack of the magnetic layer tends to be generated.
Accordingly, it would be difficult to reduce the material cost and the inertia moment by thinning the magnetic layer. Also, since the rotor body is made of metal, the rotor as a whole is heavy to bring about a large amount of inertia moment.
Further, since the magnetic layer made of rare earth plastic magnet is fixed to the rotary body with adhesives, a thickness of the adhesives would be localized to degrade the precision of coaxiality.
Also, a stepping motor in which the rotor body is made of plastic and a ferrite plastic magnet layer is fixed to an outer periphery of the rotor body has been proposed. This rotor is light in weight because the rotor body is made of plastic. However, the plastic magnets fixed to the outer circumferential surface are not made of rare earth plastic magnetic material but of ferrite plastic magnetic material. There is no consideration to reduce the amount of the material or to avoid the separation and cracks at all.